The following description concerns a procedure for pumping heat. In general, the temperature of heat is increased by using a working fluid which is desorbed at a working fluid pressure p.sub.1 from a solid adsorbent contained in at least one adsorber/generator while supplying heat at a high temperature T.sub.2, where said working fluid is transformed from the vapor to a liquid phase at an intermediate temperature T.sub.1, while liberating heat, where the liquid phase of said working fluid is transformed to a gaseous phase at low working fluid pressure p.sub.0 while taking up heat at a low temperature T.sub.0, where said gaseous working fluid is adsorbed and liberates heat in at least one further adsorber/generator containing said solid adsorbent, and where the process is continued by alternately (and shifted in phase) desorbing and adsorbing in the two or more adsorber/generators. A heat pump is a device which takes up heat at a low temperature T.sub.0 and releases heat at a higher temperature T.sub.1. In heat pumps which are based on sorption cycles, the energy input necessary for this process is supplied as heat at a third temperature T.sub.2, which is higher than T.sub.0 and T.sub.1. The heat pumping effect is achieved by the sorption or desorption of a working fluid (in general a gas) in an absorbent material which has a high affinity towards the working fluid.
Sorption heat pumps (also called chemical heat pumps) were first suggested by Altenkirch in 1910 (DE PS No. 427278). Most sorption heat pumps use a liquid as absorbent material and are called absorption heat pumps.
In the simplest system, an absorbent liquid, e.g., aqueous LiBr-solution, is concentrated by heating it to a temperature T.sub.2 (e.g. 100.degree. C.) in a generator. A condenser, which is connected to the generator, is kept at an intermediate temperature T.sub.1. Due to the heat input part of the working fluid contained in the absorbent, liquid is desorbed in gaseous form in the generator, flows to the condenser and condenses there at a vapor pressure P.sub.1 (e.g. 150 mb). The concentration of working fluid in the absorbent liquid in the generator decreases in this process. In continuous systems the concentrated solution is removed from the generator, throttled to a lower pressure and directed to the absorber. The condensed working fluid obtained in the condenser is also throttled to a lower pressure P.sub.0 (e.g. 9 mb) and flows into the evaporator. The evaporator is connected to the absorber and is maintained at a low temperature T.sub.0. The working fluid vapor present above the liquid working fluid in the evaporator moves to the absorber where it is absorbed in the absorbent liquid which is initially at low working fluid concentration. By pumping the absorbent liquid, which now has a high working fluid concentration, to the generator the cycle is closed. In order to maintain constant temperatures in all components the heat of desorption has to be added in the generator at T.sub.2, heat of condensation has to be removed in the condenser at T.sub.1 ', heat of evaporation has to be supplied at T.sub.0 in the evaporator and heat of absorption has to be removed at T.sub.1 in the adsorber.
The heat of evaporation is roughly equal to the heat of condensation and the heat of desorption is roughly equal to the heat of absorption. As a rough approximation, one can assume the heat of evaporation to be equal to the heat of absorption (in general, it is larger by a factor of about 1.1 to 1.5). Then the C.O.P. for heating can be estimated as follows: c.o.p.(HP)=ratio of input heat at high temperature T.sub.2 to the sum of the heat removed at intermediate temperatures T.sub.1 and T.sub.1 '=(1+1)/2-2. In general the heat taken up in the evaporator is ambient heat, so that the estimated heat gain at T.sub.1 (compared to the high temperature input heat) is 100%. In reality this gain is only 30% to 70% due to various losses in the cycle.
The system can be used for cooling as well. In that case the heat uptake in the evaporator is the cooling power and the heat at T.sub.1 and T.sub.1 ' is (in general) rejected to the ambient. The estimated c.o.p. for cooling is 100% while only 50% to 80% are reached in practical applications.
Absorption heat pumps are widely used for commercial air conditioning (mainly in the U.S. and Japan). The application of use of the same process for the upgrading of industrial waste heat is possible (used mainly in Japan). However the use of currently available working fluid/liquid absorbent combinations severely limit the conditions under which these systems can be applied.
The existing combinations which are thermally stable are aggressive fluids (e.g., in the combination sulfuric acid with water as working fluid), operate at high pressures and are poisonous (as water with ammonia as working fluid), or have a very limited temperature range (as LiBr-solution with water as working fluid) due to corrosion and chemical side reactions.
While the use of solid adsorbents was popular in the early days of refrigeration equipment (1920s), they were considered inferior to the liquid absorption systems, which was mainly due to a general lack of regulating equipment and slow reaction rates of the adsorbents used (e.g. CaCl.sub.2 with methanol as working fluid).
A very recent development is the use of zeolites, a natural mineral (which can also be produced synthetically) together with water as working fluid.
Certain zeolites can withstand temperatures of more than 300.degree. C. The reaction rate of zeolites with water vapor in an (airless!) container are extremely high. The further development of synthetic zeolites (which are only known for about 40 years) is to be expected with a corresponding improvement of the temperature limit (which is the highest temperature limit of the sorption pairs with the exception of sulfuric acid). Since a solid does not spill from a tank, the application of a solid adsorbent is advantageous especially where high temperatures are involved.
The following papers concern sorption heat pumps with solid adsorbents, especially with zeolite/water as adsorbent/working fluid combination: The German patent application DE No. 2939423 describes a heating system using a solid adsorption cycle with the steps generation--condensation--evaporation--adsorption (of the working fluid).
The European patent EP No. 61888 describes a resorption heat pump with two absorbent solutions (absorption loop and resorption loop) with a solid adsorbent for heat storage in a second stage.
Neither of the above mentioned papers make efficient use of the heat stored at the upper input temperature T.sub.2. A large part of this heat is used to heat the adsorbent plus the container and the heat exchangers to the input temperature T.sub.2 instead of using the major part of this heat for the generation of the working fluid vapor.
Summary of the disadvantages of existing heat pumps (including solid adsorbent heat pumps):
1. The use of LiBr-H.sub.2 O as working pair is limited to temperatures below 160.degree. C. LiBr solution becomes highly corrosive at higher temperatures which requires the use of special, expensive materials at these temperatures. Further, chemical side reactions present a problem for the system LiBr-H.sub.2 O. LiBr is a rather expensive chemical and is therefore not economical for the storage of energy since large amounts of LiBr-solutions would have to be used.
2. The use of ammonia-water as working pair is limited to temperatures under 180.degree. C. Ammonia dissociates at temperatures above 180.degree. C. Further, the pressures involved become unreasonably high. A condenser temperature of 100.degree. C. requires a pressure of 62.5 bar. Ammonia is poisonous, which limits the use of ammonia-water systems to small sizes or to special locations.
3. The use of sulfuric acid as absorbent with water as working fluid is limited to few cases where acids are handled professionally and specially trained personnel is available as in certain chemical plants. Despite the excellent performance of such systems in the experimental stage, the general use of these systems cannot be recommended.
4. Solid adsorbent systems are operated in a quasi-continuous fashion. Since a solid adsorbent cannot be pumped in a loop, several containers with solid adsorbent must be used out of phase to achieve a quasi-continuous output of heat. The adsorbent is enclosed in relatively large containers, the so-called adsorber-generators, which are used alternatively as adsorber and as generator in a cycle. The solid absorbent has to be heated and cooled in this process together with the embedded heat exchangers and the container material The application of such systems will in general be limited to pressures below atmospheric pressure.
In a system with two adsorber/generators, the adsorption capacity of each adsorber/generator has to provide output heat over one half cycle of the system. The cycling time has a lower bound due to the thermal masses involved and due to the high cost of additional heat exchanger area which is necessary for faster cycling. Therefore, the mass of adsorbent required per adsorber/generator is quite large (e.g. 800 kg for a 20 kW system). An operation above 1 bar would require large thick-walled pressure tanks. Below atmospheric pressure, the adsorber-generators can be constructed as thin-walled containers with the solid adsorbent supporting the sheet metal walls against the outside pressure. With water as working fluid, the pressure limit of 1 bar limits the condenser temperature of solid adsorbent systems to 100.degree. C.
Additionally, the application of zeolites is limited to a pressure of about 2 bar water vapor pressure due to the poor stability of most zeolites at higher pressure. The use of solid adsorbent in systems like the one described in DE No. 2939423 results in a relatively poor efficiency since the heat necessary for heating of the adsorber-generator to the temperature T.sub.2 is not used for the generation of working fluid vapor and does therefore not contribute to the heat pumping process. Instead this heat is used without heat gain at the temperature T.sub.1 (irreversible drop of temperature from T.sub.2 to T.sub.1).